In general, the degradation of a rubber product from a starting material of a natural rubber or a dienic synthetic rubber goes on in the presence of ozone whereby the surface thereof is cracked. The cracking further goes on owing to the static and dynamic stress given to the rubber product, and as a result, the rubber product is thereby broken.
For preventing generation and propagation of the cracks owing to ozone, a rubber composition containing, as an antiaging agent, an amine-based antiaging agent such as N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine or the like is applied to rubber products. For the purpose of static prevention from ozone, wax is incorporated into the rubber composition for forming a protective film on the surfaces of the rubber products.
However, though the above-mentioned amine-based antiaging agent and wax could be effective for preventing generation and propagation of cracks in the presence of ozone, those ingredients may readily move through the polymer substrate such as the rubber component or the like and may transfer onto the surfaces of rubber products, especially tires within a short period of time, and may discolor the rubber products during storage or in use thereof to worsen the outward appearance of those rubber products. Here, in case where wax transfers onto the surface, then the surface may whiten, and in case where the amine-based antiaging agent transfers on the surface, then the surface may brown.
To the above, a technique of incorporating a polyoxyethylene ether-based nonionic surfactant or a sorbitan-based surfactant into a rubber composition for tire side walls for preventing the discoloration by the amine-based antiaging agent and wax has been disclosed (PTL 1, PTL 2, PTL 3). However, according to these techniques, it is still impossible to fully prevent discoloration of rubber compositions for treads.